Ferroelectric devices and method relating thereto

ABSTRACT

The present invention relates to an electrically controllable/tunable microwave device ( 10 ) comprising a ferroelectric substrate ( 101 ) with a variable dielectric permittivity and conducting electrodes ( 102, 103 A-C), arranged on said substrate, and the capacitance of which depends on applied voltage C(V), the microwave device comprises at least two sections or parts of the substrate/electrodes for each of which different electrical field strengths are generated upon voltage application. Said generated electrical field strengths are controlled by means of the design of the device and/or the voltage application such that the slope (dC(dV) of the voltage dependence of the capacitance (C(V)) of the microwave device can be controlled.

FIELD OF THE INVENTION

[0001] The present invention relates to microwave devices, such ascapacitors, or more specifically electrically tunable capacitors(varactors), and resonators. Even more specifically it relates toferroelectric microwave devices which are electrically controllable ortunable. Particularly it relates to electrically controllable or tunablemicrowave devices comprising a ferroelectric substrate with a variabledielectric permittivity, or a variable dielectric constant, andconducting electrodes arranged on the substrate, wherein the capacitanceof the device depends on applied voltage.

STATE OF THE ART

[0002] Different kinds of microwave components are known in the art,specifically electrically controlled components based on ferroelectricsubstrates. It is known to use both thin film ferroelectric substratesand bulk material ferroelectric substrates. The dielectric permittivityof a ferroelectric material is dependent on electric field, which ischaracteristic for ferroelectric materials. In microwave devices,varactors may e.g. be used as lumped components (with dimensions smallerthan 0.1 λ, λ being the wavelength of the microwave signal) or asdistributed components (having dimensions larger larger than 0.1 λ),e.g. in phase shifters, delay lines, resonators etc. Different exampleson such devices are for example illustrated in U.S. Pat. No. 5,472,935,WO 96/42118 with priority from SE-C-506 313, which documents herewithare incorporated herein by reference thereto.

[0003] More specifically the dielectric permittivity of a ferroelectricmaterial is characterized by a non-linear dependence on the appliedelectric field. A simple mathematical model for such a dependence isshown in the article by O. Vendik, S. Zubko, in J. Appl. Phys., Vol. 82,pp. 4475, 1997 which document also is incorporated herein by reference.Since the dielectric permittivity exhibits a non-linear dependence onthe applied electric field, a similar non-linearity in the performanceof the microwave device, in which a ferroelectric substrate is used,e.g. varactors, resonators etc., will result.

[0004] One example of a ferroelectric microwave device is aferroelectric parallel plate capacitor with e.g. a Strontium Titanate(SrTiO₃) substrate. For such a parallel plate capacitor the capacitanceof the device C(V) will vary non-linearly in dependence of the appliedvoltage. The dependence is given by the relationship C=εε₀S/d, wherein Sis the area of the capacitor plate, d is the distance between theplates, E is the dielectric permittivity of the dielectric substrate(which depends on the voltage) and ε₀=8.85×10⁻¹² [F/m]. It is howeverdisadvantageous in many practical applications that the capacitancevaries non-linearly with the applied voltage. Particularly, for manypractical applications a linear, but also some other type of non-linearrelationship between the capacitance and the applied voltage, may beadvantageous, or even required.

SUMMARY OF THE INVENTION

[0005] What is needed is therefore a microwave component, particularlyan electrically controlled microwave component based or ferroelectrics,the voltage/capacitance characteristics of which can be controlled.Particularly a microwave component as referred to above is needed forwhich the dependence of the capacitance on applied voltage can becontrolled. Even more particularly a microwave device, e.g. as referredto above, is needed for which the dependence of the capacitance onapplied voltage can be linearised, or more generally controlled so as toassume a particular non-linear dependence.

[0006] Particularly a capacitor, a varactor, or a resonator, is neededwhich fulfills the above mentioned objects. Even more particularly aparallel plate (or planar) capacitor or resonator is needed throughwhich the above mentioned objects can be achieved. Particularly avaractor is needed, which can be used as a lumped component or avaractor that can be used as a distributed component, for example inphase shifters, delay lines, resonators etc.

[0007] A method is also needed through which the voltage dependence ofthe capacitance of a microwave device can be controlled, wherein themicrowave device particularly is based on, or uses, a ferroelectricmaterial as a substrate. Most particularly a method is needed throughwhich the voltage dependence of the capacitance can be a linearised ortailored to assume any desired Form of dependence.

[0008] Therefore an electrically controllable/tunable microwave deviceis provided, which comprises a ferroelectric (dielectric) substrate witha variable dielectric permittivity (constant) and conducting electrodesarranged on the substrate, wherein the capacitance of the device dependson applied voltage According to the invention the microwave device willcomprise at least two sections or two parts upon voltage applicationsuch that different electrical field strengths will be generated in thedifferent sections or parts, whereby the slope (dc/dv) of the linerepresenting voltage dependence of the capacitance can be controlled. Itmay be positive or negative, corresponding to a convex or a concaveline.

[0009] In one particular implementation the device comprises a capacitoror a varactor, or particularly a parallel plate capacitor. In anotherembodiment it comprises a resonator, particularly a parallel plateresonator. Development or generation of different electrical fieldstrengths in different sections or different parts of the microwavedevice can be provided for in different manners.

[0010] In advantageous implementations the device actually is formed ordesigned so as to comprise different sections or parts, whereby eachsection or part has a thickness and/or an area, particularly a platearea, that differ(s) from the thicknesses and/or the plate areas of theother sections (or the other section if there is only one othersection). Thus the slope of the voltage dependence (the derivative) ofthe capacitance can be controlled by appropriate selection of thethicknesses and/or the plate areas (shapes) of the different sections.

[0011] In a particular implementation, for a parallel plate device, thesame voltage is applied to the different sections or parts of thedevice, the different sections/parts being in electrical contact, andthe generated electrical field of a section or of a part will depend onthe thickness of the respective section.

[0012] Particularly there is one common, first, electrode plate and onecommon ferroelectric substrate, and for each section or part anotherseparate, second, electrode is provided. For each section the thicknessof the corresponding ferroelectric portion is different, and the secondelectrodes are provided at different distances from the common firstelectrode plate. The same voltage is applied to the different sections.Then, if the second electrodes are not electrically in contact, for eachsection a separate connection has been to be provided to each secondelectrode plate for application of the same voltage. In anotherembodiment the second electrode plates are electrically connected toeach other, but the size and/or the shapes of the sections differ. Inthat case only one connection is required and the same voltage willstill be applied to all sections but since they have thicknesses, thegenerated electrical fields will be different for each section.

[0013] Thus, the device can be so designed that it comprises only one“section” before application of a voltage, the different parts orsections actually being created through the application of a voltage.Alternatively it is so designed that different sections areextinguishable or formed by discrete different sections.

[0014] In still another embodiment the different sections are completelyseparate but have the same size and shape. In that case differentvoltages will have to be applied to the respective sections in order toenable generation of different electrical fields.

[0015] In still another implementation the sections are separate butstill differ from one another and the same voltage is applied to all ofthem. Generally the number of different sections will give the accuracyof the controllability of the slope (dc/dv) of the voltage dependence ofthe capacitance of the device. In a particular implementation bothelectrode plates are common for all parts or sections of the device, andthe ferroelectric substrate varies in thickness such that one of theplates disposed thereon will vary in shape more or less continuously,such that upon application of a voltage, the generated electrical fieldin different parts of the device will differ. In still anotherembodiment the ferroelectric substrate varies in thickness such thatboth plates disposed on either sides thereof will vary in shape, e.g. bynot being flat, such that the distance between them varies. Thevariation in thickness of the substrate may be continuous or consist ofdiscrete steps.

[0016] In particular embodiments the device is disk shaped, circular,hexagonal, ellipsoidal, rectangular or of any other appropriate regularor irregular shape. More Particularly the thinnest section or part mayhave the smallest plate area, whereby the thinnest and smallest sectionis disposed at the center of the device. In one particular embodimentthe thickness of the ferroelectric substrate is given a trapezoidalshape for providing different sections and, as referred to above, oneelectrode plate may be common, whereas the other electrode platecomprises distinct electrode plates, for defining different sections,which may be electrically connected or not. Alternatively also the othersecond, electrode plate is common for all sections but exhibits acontinuously varying shape, following the substrate surface, such thatthe device will exhibit a varying thickness. Particularly the devicecomprises a large number of different sections or parts. In alternativeimplementations it merely comprises a limited number of sections orparts, e.g. three or four or any other appropriate number.

[0017] In one particular implementation the differences in thickness andplate area from one section to another, between adjacent sections, areinfinitesimal, such that the cross-section of the microwave devicethrough the ferroelectric substrate and the common and/or sectionalelectrodes will exhibit a substantially continuously changing thicknesswith the thinnest section at the center. Particular one surface, herecalled the upper surface, of the substrate is non-planar, andsymmetrical, with respect to the center of said upper surface,two-dimensionally or radially, increases its distance from the second,electrode plate(s) towards the circumference of the device.

[0018] In an alternative implementation it comprises a planar devicewith a thin film ferroelectric substrate structure with planarelectrodes disposed on the substrate such that a gap is formed betweenthe electrodes. In that case the gap is so shaped that the dependence ofthe capacitance on the applied voltage can be controlled. A gap may beshaped so as to vary in discrete steps, thus providing the differentsections. In an alternative implementation the gap is shaped so as tovary substantially continuously. According to different implementationsa common second electrode, a ground plane, is provided, whereas in otherimplementations it is not.

[0019] The device may also, as referred to above, comprise a number ofelectrically separate sections which have different thicknesses (andplate areas), and the same voltage may be applied to the differentelectrode sections. In one embodiment the electrically separate sectionshave substantially the same shape, size and thickness and differentvoltages are applied to the different sections. Any combination is inprinciple possible, e.g. that different sections may be used anddifferent voltages may also be applied to the different sections suchthat a combination of shaping or designing the device and of applicationof different voltages is implemented. Alternatively it is merely thedesign that is made such that the voltage dependence of the capacitancecan be controlled by providing for different sections in which differentelectric field strengths are generated.

[0020] The substrate particularly comprises a substrate of SrTiO₃, of abulk material or comprising a thin film. Other alternatives are forexample BaSrTiO₃, Barium Strontium Titanate etc. The conductingelectrodes may comprise normal conductors, e.g. of Au, Al, Cu.Alternatively they comprise superconductors, e.g. made of YBCO, Nb etc.,The electrodes may also comprise low or high temperaturesuperconductors. In one particular implementation a normal conductinglayer is applied above a superconductor or a high temperaturesuperconductor. In still another embodiment a buffer layer, which isthin, may be arranged between superconducting electrodes and theferroelectric substrate.

[0021] The invention also discloses a method of controlling thevoltage/capacitance characteristic of an electricallycontrollable/tunable microwave device comprising a ferroelectric 10substrate, with a variable dielectric permittivity, and conductingelectrodes disposed on said substrate. The method comprises the stepsof; disposing the ferroelectric substrate and the electrodes andproviding for voltage application such that a different electric fieldstrength is generated for each of a number of sections or parts thusformed in the device; controlling the generation of different electricfield strengths, or a more or less continuously varying electric fieldstrength, for controlling the slope of the voltage dependence of thecapacitance.

[0022] Particularly the method comprises the steps of; designing thesubstrate and the electrodes such that different sections are formed;applying the same voltage to the different sections such that differentelectrical field strengths are generated for each of said sectionsdepending on the size and/or thickness of the sections. Even moreparticularly the method comprises the steps of, for a parallell platedevice; shaping the substrate and the electrode(s) such that a firstcentral section will have a first (plate) area and a first thickness;increasing for subsequent sections, the thickness and the area stepwiseor continuously. In particular, on one side of the substrate, a commonelectrode plate may be provided, the device constituting a parallelplate resonator or capacitor. Alternatively the substrate is providedbetween two common electrode plates of which one or both is given ashape corresponding to the surface(s) of the substrate such thatdifferent sections are formed upon application of a voltage. In analternative embodiment, it comprises the steps of; providing a number ofelectrically separate sections of the same size and shape; applyingdifferent voltages to the similarly sized and shaped sections, such thatfor each section a different electrical field is generated.Alternatively one common substrate with a varying thickness is used andone of the first and second electrodes comprises separate portions, thesame voltage being applied to each one of the sections.

[0023] In an alternative embodiment, for a planar device, the methodcomprises the steps of; designing the width of the gap between planarelectrodes disposed on a ferroelectric substrate such that uponapplication of a voltage to the electrodes, the generated electricalfield strength will vary such as to allow for controlling the slope(dc/dv) of the dependence of the capacitance on the applied voltage.

[0024] The method may in general comprises the step of designing thesubstrate and/or the electrodes such that, upon voltage application, thecapacitance varies with the applied voltage in a desired manner.Advantageously the method comprises the step of making the dependence ofthe total capacitance on the applied voltage linear, i.e. the C(V)dependence is linearised. This means that dc/dv is constant.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The invention will in the following be further described, in anon-limiting manner, and with reference to the accompanying drawings, inwhich:

[0026]FIG. 1 is a diagram illustrating the dependence of the dielectricpermittivity of a substrate of SrTiO₃, at 35° K, on applied voltage,

[0027]FIG. 2 is a diagram schematically illustrating a comparisonbetween the C(V) dependence for a varactor comprising one section andfor a linearised varactor comprising three sections according to thepresent invention,

[0028]FIG. 3A illustrates a circular parallel plate capacitor/resonatoraccording to a first embodiment of the present invention,

[0029]FIG. 3B is a cross-section of the device of FIG. 3A,

[0030]FIG. 4 is an isometrical view of a circular parallel-plate devicecomprising three sections according to another embodiment of theinvention,

[0031]FIG. 5 schematically illustrates another embodiment of a device inwhich one of the substrate surfaces, on which e.g. a common electrodeplate is disposed, is non-planar and varies continuously such that thethickness of the substrate will vary,

[0032]FIG. 6 illustrates another embodiment of a device according to theinvention wherein the substrate has a continuously varying thickness bymeans of non-planar upper and lower surfaces,

[0033]FIG. 7 illustrates very schematically an embodiment with asubstrate for which the thickness varies in discrete steps, provided forby an upper surface and a lower surface being non-planar,

[0034]FIG. 8A illustrates an embodiment of a microwave device comprisingtwo planar electrodes with a gap between them,

[0035]FIG. 8B illustrates an embodiment similar to that of FIG. 8A alsoincluding a ground plane electrode,

[0036]FIG. 8C shows a planar electrode structure with a split-upelectrode having sections of different sizes, at different distancesfrom another common electrode,

[0037]FIG. 9A schematically and for illustrative purposes shows a simpleembodiment with three separate sections of the same size and shape towhich different voltages are applied, and

[0038]FIG. 9B shows, for illustrative purposes, a device comprisingthree separate sections of different size and thickness, to whichsections the same voltage is applied.

DETAILED DESCRIPTION OF THE INVENTION

[0039] The dielectric permittivity, or the dielectric constant, of aferroelectric material depends non-linearly on the applied voltage, orrather on the resulting applied electric field. In FIG. 1 the dependenceof the dielectric constant of SrTiO₃, which is a typical ferroelectricmaterial, at a temperature of 35K, on applied voltage, is illustrated.More specifically FIG. 1 illustrates the dependence of the absolutevalue of the dielectric constant on applied voltage in the voltage range0-200V. Therefore a parallel plate capacitor made of such aferroelectric material will exhibit a similar non-linear dependence ofthe capacitance of the device on the applied voltage, as given by therelationship C=εε₀S/d, where S is the plate area of the capacitor, d isthe distance between the plates and ε is the dielectric constant(dependent on V), and ε₀=8.85×10⁻¹² [F/M].

[0040] In practical applications a linear or another type of non-lineardependence, i.e. different from the one shown in FIG. 1, between thecapacitance and the applied field or voltage, is often required.Therefore, according to the invention, a way of controlling the slope ofthe C(V) dependence, (i.e. the derivate dC/dV) is suggested.Particularly it may be linearised (corresponding to dC/dv being aconstant) and/or tailored as illustrated in the embodiments to followbelow.

[0041] According to one embodiment of present invention a microwavedevice is suggested in which for each of at least two sections or partsof the device, upon a voltage application, the generated electricalfield will be different For one and the same applied voltage thegenerated electrical field in the respective section/part will bedifferent, since the generated electrical field depends on the thicknessof the device, or the distance between the plates as E=V/d, on conditionthat the shape and/or size of the device is not the same throughout thedevice. This means that a different electric field can be generated atdifferent sections by giving the sections different shapes and/or sizes.It is also possible to apply different voltages to different sections,or to implement a combination of both, i.e. providing a device ofvarying size and/or shape and to apply different voltages to differentparts of the device. This is also schematically illustrated in FIGS. 9A,9B.

[0042] As one example, a disk shaped capacitor, or a parallell plateresonator, may be formed or designed to comprise two or more sections ofthicknesses d1, d2, d3, . . . with plate areas S1, S2, S3, . . . . Thedependence of the total capacitance of the applied voltage of such astructure can be controlled or tailored, specifically linearised, byproperly selecting the thicknesses d1, d2, d3 and the areas S1, S2, S3,. . . . of the different sections. The total capacitance of such adevice will particularly be given by:

c(E)=ε(E)ε₀ S 1/d 1+ε(E)ε₀ S 2/d 2+ε(E)ε₀ S 3/d 3+ . . . =S 1(ε(E)ε₀ /d1+ε(E)ε₀(S 2/S 1)/d 2+ε(E)ε₀(S 3/S 1)/d 3+ . . . )=S 1(ε(E)ε₀ m 1/d1+ε(E)ε₀ m 2/d 2+ε(E)ε₀ m 3/d 3+. . . ),

[0043] wherein m1(=1), m2, m3 are the ratios of the plate areas, S1/S1;S2/S1; S3/S1.

[0044] Thus the slope of the C(V) dependence can be controlled by properselection of thicknesses and plate areas.

[0045]FIG. 2 is a diagram schematically illustrating a comparisonbetween the C(V) dependence by means of the relative capacitance at agiven applied voltage in relation to the capacitance at zero bias, i.e.no applied voltage, for a one-section varactor wherein d1=d2=d3(indicated by a continuous line), and a linearised C(V) dependence for acorresponding three section varactor based on SrTiO₃ with d1=2 μm, d2=8μm, d3=20 μm, the ratios of the plate areas, see the formula above, herebeing m1=0.5, m2=7 and m3=16, (indicated by the dashed line in thefigure) i.e. in this case the relative capacitance, withC0(V,d,30)/C0(0,d,30) (unbiased) C1(v,d,30)/C1(V,d,0) (linearised)

[0046] Similar results can be obtained irrespectively of whether thedevice is circular or non-circular, e.g. rectangular, ellipsoidal etc.The ferroelectric substrate, particularly the thickness thereof, may bedesigned with different shapes, trapezoidal or other shapes, in order toprovide for a required slope of the voltage dependence of thecapacitance of the device. The principle is applicable to bulkferroelectric substrates as well as to thin film ferroelectricstructures. It is also possible to, alternatively or additionally, applydifferent voltages to different sections etc. Furthermore the concept isapplicable to parallel plate devices as well as to planar devices. Inthe following some exemplary implementations will be illustrated.

[0047]FIG. 3A illustrates a disk shaped capacitor (or parallel plateresonator) 10 comprising three different sections. Actually it iscapacitor if the dimensions are much smaller than the microwavewavelength in the dielectricum, whereas for a resonator the dimensionsshould be comparable to or larger than the wavelength of the microwavein the dielectricum. The disk shaped capacitor or parallel plateresonator 10 here comprises a ferroelectric (dielectric) substrate 101e.g. of SrTiO₃, with a bottom electrode 102 which is common for all thethree sections and three separate upper electrodes, 103A, 103B, 103Cdisposed on different parts or sections of the substrate 101. Theferroelectric substrate 101 is so designed that a first section is givenby the central part of it having a given radius and thickness, whereasthe second section is thicker and in the form of a ring, and finally thethird section is the thickest, also shaped as a circular ring. The threeupper electrodes 103A, 103B, 103C are in this embodiment notelectrically interconnected. Thus the voltage has to be appliedseparately via a lead or a conducting wire to each one of the upperelectrodes. In this embodiment the same voltage is applied to eachsection, and when the biasing voltage is applied, the dielectricconstant of the non-linear ferroelectric substrate 101 is changed and itwill be different for each section and different electric fields aredeveloped. For a resonator, the resonant frequency (and the Q-factor) ischanged. Particularly the ferroelectric substrate 101 comprises bulksingle crystal SrTiO₃. The upper electrodes have plate areas S1, S2, S3as discussed with reference to FIG. 2. The total capacitance of such astructure is given by the formula indicated above in association withFIG. 2.

[0048]FIG. 3B is a cross-sectional view of the circular parallell plateresonator/capacitor of FIG. 3A.

[0049] In one embodiment the conducting electrodes 102, 103A, 103B, 103Ccomprise a normally conducting material or a normal metal film, e.g. ofAu, Al, Cu. In other embodiments superconducting electrodes are used,particularly high temperature superconducting films, e.g. of YBCO. Alsoother superconducting materials may of course be used. In one embodiment(not shown) a normally conducting layer may be arranged above asuperconducting layer both to provide for functioning also attemperatures above Tc, which is the temperature at which thesuperconducting material becomes superconducting, i.e. such thatoperation is enabled both in a superconducting and in anon-superconducting state, and for protective purposes as a protectionlayer. Furthermore it is possible to arrange a thin buffer layer betweenthe conducting electrodes (particularly if they are superconducting) andthe ferroelectric substrate, among others in order to improve thequality of the superconducting films at the deposition stage and tostabilize the superconducting film-ferroelectric system by controllingthe chemical reactions, e.g. exchange of oxygen between thesuperconducting films and the dielectric substrate. Although in thisembodiment the same voltage is applied to each one of the sections, itwill also be possible to apply different voltages to the differentsections and thus to combine designing of the ferroelectric substrateand the application of different voltages to obtain the appropriategenerated electric fields in the respective sections to obtain thedesired slope of the voltage dependence of the capacitance.

[0050] In FIG. 4 a microwave device comprising a parallel plateresonator or a capacitor 20 is illustrated. It comprises three sections,similar to the embodiment of FIG. 3A, 3B. On one side of theferroelectric substrate 201 a bottom electrode, or more generally afirst electrode 202 is disposed, which is common for all the threesections, whereas on the too of the ferroelectric substrate 201, orrather on the opposite side of the ferroelectric substrate, a secondelectrode 203 is disposed which is given the same shape as the upper(here) portion of the ferroelectric substrate. A first (central) andcircular section has a first thickness h₁, the second section, which isring shaped, has a second thickness h₂, and the third section, whichalso is ring shaped, has a third thickness h₃, wherein h₃>h₂>h₃. Thethickness of the ferroelectric substrate 201 varies with discrete steps,the thinnest section being provided at the center of the device Thefirst section has a radius r₁, the second section defines a ring with anouter radius r₂ and the third section has an outer radius r₃, whereinr₃>r₂>r₁. Since the different portions of the upper electrode 203 inthis embodiment are electrically connected, i.e. the upper electrode 203actually comprises one common electrode, a voltage is applied to theupper electrode, i.e. to the three sections, by means of one singlelead. In this embodiment it is connected to the third section. However,in an alternative embodiment it may be connected to the first section orto the second section. Thus, in this embodiment the same voltage isapplied to all the sections, and the variation in the generatedelectrical field is given exclusively by the design of the differentsections.

[0051]FIG. 5 discloses still another example of a parallel platemicrowave device 30. A ferroelectric substrate 301 is provided with afirst electrode 302 and a second electrode 303 on opposite sidesthereof. It should be clear that upper and lower are here merely aredenotations given for explanatory reasons. The circular ferroelectricsubstrate is flat on one side. The other (upper) side or surface is soshaped such that the substrate thickness will be smallest at the center.For a circular device the substrate has a radius r, and thus thethickness will be a function of the distance from the center, h(r). Thusin this embodiment there are no distinct sections, but at theapplication of a voltage to the electrodes 302, 303, the generatedelectrical field will be different in different parts of the device,even if the same voltage is applied to each section or to each part. Dueto the variation in thickness of the ferroelectric substrate, thegenerated electrical field will vary throughout the device, and it canbe controlled such that the required or wanted slope of the dependenceof the capacitance of the device can be obtained by appropriatelyselecting the design of the ferroelectric substrate.

[0052]FIG. 6 shows still another example of a parallel plate microwavedevice 40 with a ferroelectric substrate 401 with a varying thickness.It is also here supposed that the device has a circular shape and thethickness of the ferroelectric substrate h depends on the radius, h(r),i.e. it varies with the distance from the center. Both the upper and thelower surface of the ferroelectric substrate vary continuously in asimilar way, e.g. assuming the shape of each a bowl, which bowls faceoutwards in opposite directions, such that the thickness of the deviceincreases towards the periphery of the device, the angle of inclinationof the bowls being such that the required slope of the C(V) dependenceis obtained.

[0053]FIG. 7 illustrates still another implementation of a ferroelectricsubstrate 501 with a varying thickness. The thickness varies stepwise ina symmetrical manner on both sides and common electrodes 502, 503 areprovided on either side of the substrate. The thickness varies indiscrete steps such that three different sections are provided with eacha given diameter and a given thickness. The electrodes are common forthe three sections, and one and the same voltage is applied to eachsection. The variation in size and shape of the ferroelectric substrate,or more generally of the different sections, gives the differentelectric fields that are generated upon voltage application. It shouldbe clear that also in this embodiment separate electrodes could beprovided for each section, i.e. electrodes which are not electricallyinterconnected, such that separate leads or conducting wires arerequired for voltage application. Still further the same voltage may beapplied to each section. Alternatively different voltages are applied tothe different sections as discussed above to take advantage both ofdifferences in design and in applied voltage for controlling purposes.

[0054] In FIG. 8A a microwave device 60 comprising a thin filmferroelectric substrate 601 is disclosed. Two planar electrodes 603A,603B are disposed on the ferroelectric substrate 601. The planarelectrodes are provided with protruding oppositely directed portions603A′, 603B′ between which a gap is formed. The protruding portions areso formed that the width of the gap and hence the generated electricfield will vary, such that a desired slope of the C(V) dependency can beobtained.

[0055]FIG. 8B shows a structure 70 similar to that of FIG. 8A butincluding the provisioning of a ground electrode 602.

[0056]FIG. 8C illustrates very schematically still another alternativeembodiment of a planar structure 80 with a thin film ferroelectricsubstrate 801 on which electrodes 803A, 803B, 803C and 804 are provided.The sizes (and shapes) of the electrodes 803A, 803B, 803C differ.Electrode 803A is the smallest, followed by electrode 803B which islarger than 803A but smaller than the consecutive electrode 803C, suchthat also in this case, like in the embodiments of FIGS. 8A, 8B,different electrical fields are developed between the respectiveelectrodes 803A, 803B, 803C and electrode 804 since the dielectricconstant of the ferroelectric material will differ from one part of theplanar electrode structure to another, and since the capacitance of eachrespective part of the device depends on the dielectric constant of thatpart, also the capacitances will differ. By carefully selecting thesizes/shapes of the electrodes, and thus the distances betweenelectrodes, the voltage/capacitance characteristics can be controlled,or particularly linearised corresponding to the derivative dC/dV being aconstant.

[0057]FIG. 9A is an explanatory figure which shows another simplifiedembodiment in which three separate parallel plate resonators orcapacitors are provided, or rather a resonator or a parallel platecapacitor split up into three different sections with respectiveferroelectric substrates 901A, 901B, 901C with electrodes 903A, 903B,903C and 902A, 902B, 902C disposed on either sides thereof respectively.It is supposed that the size and shape of each parallel plate resonatorsection is the same as the shape of the others and they also have thesame thickness. In order to be able to control the capacitance of eachpart or substructure, different voltages thus have to be applied to thedifferent substructures. It is here supposed that a voltage V91 isapplied to the structure 900A, a voltage V92 is applied to structure900B and finally V93 is applied to structure 900C, whereby V91<V92<V93.Since the dielectric constant of the respective substructure depends onthe generated electrical field of the respective structure(E91<E92<E93), the dielectric constant of substructure 900A will belarger than that of substructure 900B which in turn will be larger thanthat of substructure 900C. Correspondingly the capacitance C₁₁ ofsubstructure 900A will be higher than that, C₁₂, of substructure 900Bwhich in turn is higher than C₁₃ of substructure 900C. It should beclear that, in order to obtain the same effect, it is possible to givethe different substructures slightly different thicknesses but stillapply different voltages to them. The voltage differences can then besmaller. It is also possible to merely work with the thicknesses and/orshapes, in which case the same voltage can be applied to eachsubstructure. By particular selections of thicknesses/shapes andvoltages, other relationships than those referred to above can also beobtained. Such an embodiment is illustrated in FIG. 9B. The parallelplate resonator or capacitor structure 90B is divided into threesubstructures, wherein substructure 900D is larger and thicker thansubstructure 900E, which in turn is larger and thicker than substructure900F. Also in this embodiment each substructure comprises aferroelectric substrate 901D, 901E, 901F respectively, on either sidesof which electrodes 9020, 903D; 902E, 903E; 902F, 903F respectively aredisposed. It is supposed that one and the same voltage V90 is applied toeach one of the substructures. Depending on how the plate areas and thethicknesses of the structures are selected, the electric field generatedin substructure 900D, 900E and 900F respectively can be controlled,which in turn affects the dielectric constant. By selecting therespective areas and thicknesses of the substructures, the slope of theC(V) dependence can be controlled or tailored as desired by changing oraffecting the partial contribution of the respective capacitance of therespective different substructures.

[0058] It should be noted that the invention of course not is limited tothree substructures, but this is merely shown as an example. In generalthe slope of the C(V) dependence (dC/dV) can be controlled morecarefully the more sections or parts there are, and each structure onlydiffering to a minor extent from the adjacent structures as alsodescribed more in detail above.

[0059] It should be clear that also other ferroelectric or tunabledielectric materials than SrTiO₃ can be used. It should be clear thatalso in other aspects the invention is not limited to the specificallyillustrated embodiments, but that it can be varied in a number of wayswithin the scope of the appended claims

1. Electrically controllable/tunable microwave device(10;20;30;40;50;60;70;80;90A;90B) comprising a ferroelectric substrate(101;201;301;401;501;601;801;901A-C;901D-F) with a variable dielectricpermittivity and conducting electrodes(102,103A-C;202,203;302,303;402,403;502,503;603A,A′,603B,B′;803A-C,804;902A-C,903A-C;902D-F,903D-F) arranged on saidsubstrate, and the capacitance of which depends on applied voltage C(V),characterized in that the microwave device comprises at least twosections or parts of the substrate/electrodes for each of whichdifferent electrical field strengths are generated upon voltageapplication, wherein said generated electrical field strengths arecontrolled by means of the design of the device and/or the voltageapplication such that the slope (dC(dV) of the voltage dependence of thecapacitance (C(V)) of the microwave device can be controlled.
 2. Amicrowave device (10;20;30;40;50;90A;90B) according to claim 1,characterized in that it comprises a parallel plate resonator.
 3. Amicrowave device (10;20;30;40;50;90A;90B) according to claim 1,characterized in that it comprises a parallel plate capacitor, or avaractor.
 4. A microwave device (10;20;30;40;50;90A;90B) according toany one of claims 1, 2 and 3,: characterized in that each section has athickness and/or an electrode plate area differing from thethickness(es) and/or the plate area(s) of the other section(s), and inthat the dependence of the capacitance on the voltage is controlled byappropriate selection of the thicknesses and the electrode plate areasof the sections.
 5. A microwave device (10;20;30;40;50;90B) according toclaim 4, characterized in that the same voltage is applied to thedifferent sections and in that, for the applied voltage, the generatedelectric afield of a section depends on the thickness and the plate areaof the section.
 6. A microwave device (10) according to claim 5,characterized in that one common electrode plate (102) and one commonferroelectric substrate (101) are provided, and in that for each sectionseparate, second electrodes (103A,103B,103C) are provided, that for eachsection the thickness of the corresponding ferroelectric substrateportion is different, and in that the second electrodes (103A,103B,103C)are provided at different distances from the common first electrodeplate (102).
 7. A microwave device according to claim 5 or 6,characterized in that it is disk shaped, circular, ellipsoidal orrectangular and in that the thinnest section (103A) has the smallestplate area (103A), said thinnest and smallest section being disposed atthe center of the microwave device (10).
 8. A microwave device accordingto any one of claim 5-7, characterized in that the thickness of theferroelectric substrate is given a trapezoidal shape for providing thedifferent sections.
 9. A microwave device according to any one of thepreceding claims, characterized in that is comprises a large number ofdifferent sections and in that the accuracy of the controllability ofthe slope (dC/dV) of the line representing the dependence of thecapacitance on the voltage, increases with the number of sections.
 10. Amicrowave device (10;20;30;40;50) according to claim 9, characterized inthat the differences in thickness and plate area from one section toanother, between adjacent sections, are infinitesimal, such that thecross-section of the microwave device through the ferroelectricsubstrate and the common and/or the sectional electrodes exhibits asubstantially continuously changing thickness with the thinnest sectionat the center.
 11. A microwave device (60;70;80) according to claim 1,characterized in that it comprises a thin film ferroelectric substrate(601:801) structure with planar electrodes (603A,A′;6038,B′;803A-C,804)disposed on the substrate (601;801) such that a gap is formed betweensaid electrodes, and in that the gap is so shaped that the slope of thedependence of the capacitance on applied voltage (dC/dV) can becontrolled.
 12. A microwave device (80) according to claim 11,characterized in that the gap is shaded so as to vary in discrete steps,thus providing different sections.
 13. A microwave device (60;70)according to claim 11, characterized in that the gap is shaped so as tovary substantially continuously
 14. A microwave device (90A;90B)according to claim 1, characterized in that it comprises a number ofelectrically separate sections.
 15. A microwave device(10;20;30;40;50;60;70;80;90A;) according to any one of the precedingclaims, characterized in that the generation of different electricfields in different parts/sections or of a continuously varying electricfield exclusively is given by the design of the microwave device, i.e.of the ferroelectric substrate and/or the conducting electrodes.
 16. Amicrowave device (10;20;30;40;50;90B;) according to claim 15,characterized in that the sections have different thicknesses and plateareas and in that the same voltage is applied to the different electrodesections.
 17. A microwave device (90A) according to any one of claims1-14, characterized in that the generation of different electric fieldsin different parts/sections or of a substantially continuously varyingelectric field is given exclusively by application of different voltagesto different parts/sections.
 18. A microwave device (90A) according toclaim 17, characterized in that the electrically separate sections havesubstantially the same shape, size and thickness, and in that differentvoltages are applied to different sections.
 19. A microwave device(10;20;30;40;50;90A;90B) according to any one of claims 1-10, 14-16,characterized in the substrate comprises a bulk ferroelectric substrate,e.g. of SrTiO₃.
 20. A microwave device according (60;70;80) to any oneof claims 11-13, characterized in that the substrate comprises a thinfilm ferroelectric substrate, e.g. of SrTiO₃.
 21. A microwave device(10;20;30;40;50;60;70;80;90A;90S;) according to any one of the precedingclaims, characterized in that it is so shaped and/or (a) voltage(s)is/are so applied that the slope of the dependence of the capacitance onvoltage will be linear dC/dV being constant.
 22. A method of controllingthe voltage/capacitance (V/C) characteristic of an electricallycontrollable/tunable microwave device comprising a ferroelectricsubstrate, with a variable dielectric permittivity and conductingelectrodes disposed on said substrate, characterized in that itcomprises the steps of: disposing the ferroelectric substrate and theelectrodes such that the device comprises at least two sections orparts, providing for voltage application to the sections or parts suchthat a different electric field strength is generated for each of thesections or parts of the device, or that a substantially a continuouslyvarying electric field is generated, for controlling the slope (dC/dV)of the voltage dependence of the capacitance.
 23. The method of claim22, characterized in that is comprises the steps of: designing thesubstrate and the electrodes such that differently sized and/or shapedsections are formed, applying the same voltage to the different sectionssuch that different electric field strengths are generated for each ofsaid sections.
 24. The method of claim 23, characterized in that itcomprises the steps of: designing the substrate and the electrodes suchthat a parallell plate device is formed and such that a first sectionhas a first (plate) area and a first thickness, and for subsequentsections the thickness and the area increases stepwise or continuously.25. The method of claim 24, characterized in that on one side of thesubstrate a common electrode plate is disposed and in that the devicecomprises a parallel plate resonator or capacitor.
 26. The method ofclaim 25, characterized in that it comprises the steps of: providing anumber of electrically separate sections of the same size and shape,applying different voltages to the similarly shaped sections such thatfor each section a different electrical field is generated.
 27. Themethod of claim 22, characterized in that it comprises the steps of:designing the width of a gap between planar electrodes disposed on theferroelectric substrate such that upon application of a voltage to theelectrodes the electrical field strength varies continuously or indiscrete steps to control the slope (dC/dV) dependence of thecapacitance on the applied voltage.
 28. The method of any one of claims22-27, characterized in that it comprises the step of: designing thesubstrate and/or the electrodes, such that upon voltage application thecapacitance varies linearly with the applied voltage, i.e. dc/dV isconstant.
 29. A method of claim 25, characterized in that it comprisesthe steps of: designing the ferroelectric substrate and/or electrodesand/or providing for voltage application to the electrodes such thatdifferent electric fields are generated in different parts/sections ofthe device, or such that a continuously varying electric field isgenerated, wherein the electric field strength(s) are controlled fordetermining the slope (dC/dV) of the line representing the dependence ofthe capacitance on the voltage.